Electric plug connectors for thermocouple elements of the kind described are electrically connected to the electrical conductor ends of a thermocouple or to a compensating conductor attached thereto, usually by means of a clamp connection. Plug connectors of this kind for thermocouples are also referred to as thermocouple plugs or thermoplugs. They are used as a universal electrical connecting means and for the standardized electrical connection of thermocouples to measuring instruments, measuring cabinets, electrical circuit arrangements or the like when constructing a temperature measuring chain. Thermocouple plugs are usually plugged into a socket or into a plug-in coupling that is complementary to the plug in order to make contact with a measuring instrument or with a circuit arrangement. For this purpose, the plug-in or coupling socket to be connected to the thermocouple plug is mounted either directly on the board of a circuit arrangement, for example an electrical printed circuit board, or on a housing of the measuring instrument, for example a measuring or switching cabinet. In order to cover larger measuring distances, a compensating conductor, which, in a certain temperature range, has comparable thermoelectric properties to the two thermocouple conductors, can be used as a cost-effective extension between the temperature measuring point and a measuring instrument. If a compensating conductor is used, then this is usually provided with a connecting terminal to which the two open thermocouple conductors can be electrically connected, wherein the thermocouple plug is fitted to the opposite end of the compensating conductor.
The measuring principle of thermocouples for measuring temperature is based, according to the thermoelectric effect, on the development of a thermoelectric voltage between two wire conductors that are connected to one another at one end when the connecting point has a different temperature from the two open conductor ends at which the thermal voltage can be measured. The two electrical conductors, which are made of different materials and are referred to as a thermocouple pair, are connected to one another at the temperature measuring point of the thermocouple. At the open end of each, the so-called comparison or cold junction, the two thermocouple conductors are connected to a measuring instrument for measuring the thermoelectric voltage.
In order to determine the temperature of the measuring point from the value of the measured thermoelectric voltage by means of a thermocouple, the temperature of the comparison point must be taken into account, as the thermoelectric voltage that can be measured at a thermocouple depends on the temperature difference between the measuring point and the comparison point. Consequently, the temperature of the measuring point must be determined from the measured value of the thermoelectric voltage relative to the temperature of the comparison point.
Temperature-dependent resistors, which are used for determining and compensating the temperature of the comparison point of a thermocouple, are disclosed in the prior art. Furthermore, methods that not only determine the values of the thermoelectric voltage by means of integrated circuits but also directly compensate the comparison temperature are disclosed in the prior art. The disadvantage of known methods of this kind is, however, that the temperature is not measured directly at the comparison point of the thermocouple, that is to say not at the two conductor ends of the thermocouple or the compensating conductor, but usually on a circuit arrangement or a board that is located within a housing of a measuring instrument or a measuring cabinet. As the two conductors are usually attached to a terminal outside the measuring instrument, a temperature variation therefore exists between the actual and the measured temperature of the comparison point.
A universal connector with temperature compensation of the temperature of the comparison point is disclosed in U.S. Pat. No. 5,167,519. A thermocouple plug can be electrically connected to such a universal connector by means of a screw terminal connection. In order to measure or compensate the temperature of the comparison point, an electrical temperature sensor is fitted on universal connectors of this kind between two galvanically isolated copper blocks, to which the connecting pins of a thermocouple plug to be connected can be fixed by means of a clamp connection.
The universal connector disclosed in publication U.S. Pat. No. 5,167,519 does not solve the problem that the electrical connecting point between a thermocouple plug or between its electrical connecting pins and a universal connector equipped with a temperature sensor presents a not inconsiderable thermal contact resistance.
As a temperature gradient is necessarily present between the measuring point of the thermocouple, that is to say at the measuring location, and the comparison point, that is to say at the conductor ends of the thermocouple that are electrically connected to the thermocouple plug of a thermocouple, the comparison point is subject to a continuous supply or removal of heat and therefore to at least temporary heating and cooling—depending on whether a positive or negative temperature gradient is present—due to the heat-conducting properties of the two thermocouple conductors.
Furthermore, a continuous supply or removal of heat can likewise take place via the connecting pins of the thermocouple plug, which, for example is connected to a universal plug according to publication U.S. Pat. No. 5,167,519. If, however, the connecting point between the electrical connecting pins of the thermocouple plug and the universal connector has a thermal contact resistance, then a temperature gradient can be observed between the temperature of the comparison point and that at the temperature sensor, as a result of which the measurement of the comparison temperature is subject to a measuring error.
If, in addition, thermocouple plugs with thermocouple conductors of different cross sections or thermal conductivities are fitted to a universal connector of this kind, then an effect on the temperature measurement at the comparison point is likewise to be expected as a result of a changed thermal conduction resistance and a consequently changed temperature gradient.
Furthermore, if a thermocouple plug fitted to a universal connector of this kind is subjected to hot or cold air currents associated with locally different ambient temperatures, or if a thermocouple with a starting temperature that differs from the temperature at the temperature sensor is connected to a universal connector of this kind, for example when changing or replacing a thermocouple, then a temperature gradient is likewise present between the temperature sensor and the comparison point, as a result of which the measuring chain is subject to a random or dynamic measuring error respectively.